Troll Bridge Ambush (5e Quest)
From D&D Wiki
Encounter takes place while the party is nearing a bridge over a small river. The intention is to surround them on the bridge. Level 6-8
DC 12 perception check will spot the troll under the bridge. The Bridge Troll has normal troll stats, but resists fire. The players can avoid combat by paying a toll to the troll. Fee is up to the GM.
If the party crosses the bridge without spotting the troll, six goblins will emerge from the woods across the bridge, and the troll will climb up behind the party, trapping them on the bridge. Each time a player is hit by the troll they must roll a dc 15 Dex/athletics/acrobatics check to avoid being knocked into the river. The river is 15 feet deep at it's center, and slow moving, so swim checks don't need to be tough.